Activating a [FeFe] Hydrogenase Mimic for Hydrogen Evolution under Visible Light

Abstract Inspired by the active center of the natural [FeFe] hydrogenases, we designed a compact and precious metal‐free photosensitizer‐catalyst dyad (PS‐CAT) for photocatalytic hydrogen evolution under visible light irradiation. PS‐CAT represents a prototype dyad comprising π‐conjugated oligothiophenes as light absorbers. PS‐CAT and its interaction with the sacrificial donor 1,3‐dimethyl‐2‐phenylbenzimidazoline were studied by steady‐state and time‐resolved spectroscopy coupled with electrochemical techniques and visible light‐driven photocatalytic investigations. Operando EPR spectroscopy revealed the formation of an active [FeIFe0] species—in accordance with theoretical calculations—presumably driving photocatalysis effectively (TON≈210).

Steady-state spectroscopy: UV-vis absorption spectroscopy was carried out with a V-780 Spectrophotometer (JASCO) and a SPECORD S600 (Analytik Jena). Emission spectroscopy and emission quenching experiments were performed with a FLS980 Spectrometer (Edinburgh Instruments), absolute fluorescence quantum yields were determined with an FLS980 Integrating Sphere (Edinburgh Instruments).

Time-resolved spectroscopy:
Streak camera -The two-dimensional (time and wavelength) emission decay profiles were measured using a Hamamatsu streak scope C4334 in photon counting mode using a time window of 500 ns. After excitation with the frequency-doubled output of a Ti-sapphire laser (Tsunami, Newport Spectra-Physics GmbH; pulse-to-pulse repetition rate 400 kHz after passing a pulse selector, model 3980, Newport Spectra-Physics GmbH) at ex = 460 nm, the luminescence of the sample was collected in a 90°-arrangement.
Nanosecond transient absorption -The excitation light source for measurements on the submicro-second time scale is based on a Nd:YAG laser (Continuum Surelite), yielding 5 ns pulses at 1064 nm with a repetition rate of 10 Hz. The fundamental pump pulses were frequency-tripled (yielding 355 nm), which was used as input for generating pump pulses centered at 420 nm using an optical parametric oscillator (Continuum Surelite). The kinetics were studied using a probe light provided by a 75 W xenon arc lamp, which was focused on the sample by a concave mirror. After passing through the sample, the probe pulses were spectrally dispersed (Acton Princeton Instrument 2300), detected on a photomultiplier (Hamamatsu R928), and processed (Pascher Instruments AB). TA kinetics were detected as a single-wavelength kinetic between 380 and 800 nm in steps of 10 nm. For all measurements, the power of the pump beam was kept at 0.35 mJ. A long pass filter (435 nm) to eliminate the pump scattering was used at wavelengths above 430 nm. 90 mm 90 mm 46 mm 5 EPR: A X-Band ELEXSYS E500 spectrometer from Bruker with HE-flow cryostat equipped with a MD5 resonator at 5 K was used for EPR measurements. Illumination was carried out at room temperature at 420 nm. The samples were frozen under illumination. All experiments were carried out in an EPR tube for low temperature measurements (3 mm diameter). Simulation of experimental data was carried out with EasySpin in MatLab. [6] Electrochemistry: Cyclic voltammetry (CV) -Basic CV was accomplished with a three-electrode setup using a glassy carbon (d = 1.6 mm) working electrode, a Ag + /Ag (in acetonitrile) reference electrode and a Pt (wire) counter electrode, driven by a Reference 600 Potentiostat/Galvanostat/ZRA from Gamry Instruments.
Spectroelectrochemistry (SEC) -The measurements were executed with a BioLogic SP-50 Potentiostat. IR SEC was carried out in a Specac Omni-Cell with a Pt (mesh) working and counter electrode and an Ag (wire) reference electrode using a Tensor 27 FT-IR spectrometer. UV-vis SEC was done with a glassy carbon working electrode, a Pt (wire) counter electrode and an Ag (wire) reference electrode making use of a SPECORD S600 UV-vis spectrometer. The glassy carbon electrode has a hole cut in to ensure maximum concentration of the desired compound during measurements.
Redox potentials are roughly recalculated against Fc + /Fc from original data by subtracting -0.63 V a [13] (published vs. NHE) or -0.38 V b [13] (published vs. SCE). [12] Electron  Figure S3: a) UV-vis spectroscopic monitoring of a photocatalytic experiment (10 µM PS-CAT and 1000 eq BIH in CH3CN/NMP = 5:1, V = 1 mL) over a period of 17 h. During the UV-vis measurement the stirrer and the LED light (455 nm) were turned off. b) UV-vis spectroscopic monitoring of the dark and oxygen quenching process for the newly formed species. c) UV-vis spectroscopic monitoring of the photostability of PS-CAT in CH3CN/NMP = 5:1 upon irradiation (455 nm). All measurements were carried out in a 1 cm x 0.5 cm inert cuvette. After irradiation of the narrow cuvette side (LED power ≈ 180 mW), the UVvis measurement was executed throughout the wide cuvette side.

Fluorescence quenching in presence of BIH
Emission quenching experiments were performed to investigate potential interaction between PS and PS-CAT with the quencher BIH. The quenching experiments were performed by adding increasing equivalents of BIH to a solution of PS and PS-CAT in a titration experiment. Not only changes in emission intensity (and spectral shape of the emission spectra) have been followed, but also in parallel absorption spectra of the solutions with increasing BIH concentration were collected during this experiment. Figure S4: a) Absorption spectra of PS (c = 6.63·10 -7 mol/L) and b) emission spectra of PS upon addition of equivalents of BIH in dry and deaerated acetonitrile. All spectra are corrected for dilution effects occurring upon addition of increasing amounts of BIH. Absorption spectra were recorded for each concentration before and after the respective emission measurements to observe potential changes in the absorption spectrum during this time. For chosen concentrations several emission spectra were recorded sequentially to test for changes in emission intensity with time. c) Development of absorption spectra after addition of BIH. d) Respective plot of I0/I vs. the concentration of BIH illustrating the changes in emission intensity in the presence of increasing amounts of BIH. For selected concentrations several sequential emission spectra were recorded. The repeated measurements after few minutes are represented by red circles.
For PS we noticed that in the solvent mixture (CH3CN/NMP = 5:1) applied in the catalytic experiment, PS apparently is lacking necessary stability, which shows in a decrease of absorbance and decreasing emission intensity with time, even without addition of BIH or additional irradiation. This behaviour is induced by the presence of NMP and the experiments for PS were performed in pure acetonitrile, where PS was stable. Upon addition of BIH, at low equivalents, we observe negligible quenching up to 200 equivalents. Only in the presence of 500 and 1000 equivalents the emission intensity of PS is significantly decreased, and the extent of quenching increases with time Absorbance Wavelength / nm PS + BIH 0 eq before 0 eq after 5 eq before 5 eq after 10 eq before 10 eq after 20 eq before 20 eq after 50 eq before 50 eq after 100 eq before 100 eq after 200 eq before 200 eq after 500 eq before 500 eq after 1000 eq before 1000 eq after as shown by sequentially collected emission spectra at the same concentration. This correlates with slight changes in the absorption spectra which we recorded in parallel. A decrease of the absorption band with time after addition of BIH can be observed. In our series of experiments, we recorded absorption spectra before and after each emission measurement at a certain BIH concentration and we performed an additional experiment upon addition of 1000 equivalents following the development of a time range of longer than 100 minutes. After this time scale the absorption spectra is changed significantly, represented by a loss of absorbance of the visible absorption band.
Whether this effect is caused by an aggregation effect or by a reaction between PS and BIH, which is changing the chromophore, is not clear at the current state of research. Under these conditions an evaluation of the quenching experiment following classical Stern-Volmer formalism is not possible.  PS-CAT shows a (quasi-)reversible reduction wave at E1/2 Red1 = -1.61 V in CH2Cl2 ( Figure S9a), which is assigned to the reduction of [Fe I Fe I ] to [Fe I Fe 0 ] and similar to previous reported complexes in this solvent. [10,14] The current function Ipc / (cν 1/2 ) for different scan rates (0.2 ≤ ν ≤ 20 Vs -1 ) ( Figure S9b)) reveals a one-electron transfer for this process, based on a comparison between a one-[(pdt)Fe2(CO)6] [15] (pdt = propane-1,3-dithiolate) and a twoelectron [(bdt)Fe2(CO)6] [16] (bdt = benzo-1,3-dithiolate) transfer reference. This is underlined by the observed vibrational shift of the original ν(CO) (2072, 2033, 2000 and 1988 cm -1 ) upon electrochemical CAT reduction ( Figure   S10). Here, i.e., the IR bands at 2072 and 2033 cm -1 presumably are shifted to 2020 and 1970 cm -1 , which is of a similar order of magnitude as for the one-electron reference [(pdt)Fe2(CO)6] (shift by ca. 70 cm -1 upon electrochemical reduction). [15] As opposed to this, the two-electron reference [(bdt)Fe2(CO)6] shows shifts in the region of 110-140 cm -1 , respectively. [16]   UV-vis SEC (Figure S11/S12) was achieved by using a UV-vis SEC cuvette with a 1 mm pathway equipped with a glassy carbon electrode as the working electrode, platinum wire as the counter electrode and silver wire as the reference electrode. The glassy carbon electrode has a hole in the center of the measurement window to ensure a high concentration of oxidized and reduced species is available for the measurement.  Wavelength / nm  Figure S12: UV-vis SEC of PS-CAT. a) Reduction. b) differential spectrum of reduction. c) Oxidation. d) differential spectrum of oxidation.

Absorption
Wavelength / nm

Computational details
All quantum chemical calculations determining structural and electronic properties of PS-CAT were performed using the Gaussian 16 program. [17] To reduce the computational demand without affecting the photophysical properties of the present photocatalyst, the terminal alkyl groups of the thiophene-based chromophore were approximated by methyl groups. The fully relaxed equilibrium geometry of PS-CAT was obtained within the singlet ground state at the density functional level of theory (DFT) by means of the B3LYP [18] XC functional. The def2-SVP [18] basis set was applied for all atoms. A vibrational analysis was carried out to verify that a minimum on the potential energy (hyper)surface (PES) was obtained. To correct for the lack of anharmonicity and the approximate treatment of electron correlation, the harmonic frequencies were scaled by the factor 0.97. [19] Subsequently, excited state properties such as excitation energies, oscillator strengths and electronic characters were calculated within the Frack-Condon (FC) structure at the time-dependent DFT (TDDFT) level of theory.
Therefore, the 100 lowest singlet and the 100 lowest triplet excited states were calculated, while the same XC functional and basis set were applied as for the preceding ground state calculations. Several computational as well as joint spectroscopic-theoretical studies on structurally related [FeFe]-hydrogenase mimics showed that this computational protocol enables an accurate prediction of ground and excited states properties with respect to experimental data, e.g. structural and electrochemical properties as well as with respect to UV-vis absorption. [20] In particular, a reliable description of the singlet-triplet splitting is of uttermost importance to elucidate excited state relaxation processes for hydrogenase mimics. State-of-the-art ab initio simulations, e.g. via CASPT2, RASPT2 and CCSD(T), show that B3LYP is able to predict such singlet-triplet splitting with reasonable accuracy for structurally related hydrogenase mimics, [21] while protonation of the active site may hamper an accurate description at the B3LYP level of theory. [22] Such protonated species were not investigated in the present study. Several theoretical studies addressing the photophysics of the active site of such [FeFe]-hydrogenase models point to a dependency of the amount of exact exchange on the excited state properties of interest. However, in case of transition metal complexes, e.g. PS-CAT, [20a, 23] a balanced descriptionas provided by the present computational setupis essential that allows to investigate the excited state properties of the photosensitzer, of the catalytic center as well as the intramolecular interactions among both moieties. [24] The extraordinary rich photophysics and photochemistry of such transition metal complexes originates from the manifold of excited states involved, i.e., metal-to-ligand charge transfer (MLCT), ligand-to-metal charge transfer (LMCT), ligand-to-ligand charge transfer (LLCT), intraligand charge transfer (ILCT), intra-ligand (IL) and metal-centered (MC) character. The present computational protocol, as provided by the present computational setup, is essential to assess the photophysics of transition metal complexes. Effects of interaction with a solvent (tetrahydrofuran, THF: ε = 7.4257, n = 1.407) were taken into account on the ground and excited states properties by the solute electron density (SMD) variant of the integral equation formalism of the polarizable continuum model. [25] The non-equilibrium procedure of solvation was used for the calculation of the excitation energies within the Franck-Condon point, which is well adapted for processes where only the fast reorganization of the electronic distribution of the solvent is important. All calculations were performed including D3 dispersion correction with Becke-Johnson damping. [26] Furthermore, excited state relaxation pathways associated to photo-induced electron transfer as well as energy transfer were studied. Therefore, fully relaxed equilibrium structures were obtained at the DFT and TDDFT for prominent triplet states, involved in the electron and energy transfer channels. Starting from the FC point, the triplet ground state (T1) of PS-CAT was optimized at the DFT level of theory. This metal-centered ( 3 MC) state features one unpaired electron in the FeFe orbital and one unpaired electron in the FeFe * orbital. Thus, the Fe-Fe bond order is decreased from one to zero upon population transfer from S0 to T1. This decreased bond order is well reflected Figure S13: Experimental (grey) and simulated (black) UV-vis absorption spectrum of PS-CAT in THF. The nature of electronic transitions (into S1 and S2) is illustrated by charge density differences (CDDs); charge transfer takes place from red to blue. Table S5: Excitation energies (ΔE), excitation wavelengths (), oscillator strengths (f) and electronic character of spin-allowed singlet-singlet and spin-forbidden singlet-triplet excitations within the equilibrium structure of the singlet ground state (S0, Franck-Condon (FC) point) as well as spin-allowed doublet-doublet excitations within the equilibrium structure of the doublet ground state of the singly reduced species. Experimental excitation wavelengths (exp) are assigned.  Figure S14: Simulated UV-vis absorption spectrum of the non-reduced singlet (black) and the singly reduced doublet (blue) species of PS-CAT in THF. The spin density of the doublet ground state (D0) show that single reduction occurs at the [FeFe]center, i.e. population of the σFeFe * molecular orbital. The nature of the D0→D25 transitions is illustrated by a charge density difference (CDD); charge transfer takes place from red to blue. Table S6: Vibrational normal modes associated to the vibrational structure in the electronic absorption spectrum of PS-CAT in Figure 3a). All vibrational modes (modes 162 to 165) in vicinity of the measured vibrational feature (~1250 cm -1 ) correspond to CC stretching and CH bending accounting for the altered electronic structure of the aromatic system upon ππ* excitation. All frequencies were scaled by a factor for 0.97.
mode 162 (1243 cm -1 ) mode 163 (1249 cm -1 ) mode 164 (1253 cm -1 ) mode 165 (1264 cm -1 ) Figure S15: Simulated transient absorption spectra of PS-CAT. Positive signals (excited state absorption) originate from spinallowed triplet-triplet excitations within the equilibrium structures of 3 ππ* (red) and 3 MC (blue), respectively. Negative signals (ground state bleach) stem from spin-allowed singlet-singlet excitations (black) within the equilibrium structure of the singlet ground state. Excitations of interest are labelled in red and blue accordingly. Spin densities of the triplet ground states (T1) obtained within 3 ππ* (red) and 3 MC (blue) and charge density differences (CDDs) illustrating the respective electronic transitions are shown; charge transfer takes place from red to blue. Table S7: Relative energies and charge density differences of all states involved in the proposed excited state relaxation scheme, see Figure 3. Energies are given with respect to the singlet ground state (S0) in the Franck-Condon point (FC, S0 equilibrium structure). Charge transfer takes place from red to blue.